Phytotoxicity of Products from Rhizospheres of a Sorghum-Sudangrass Hybrid (Sorghum bicolorXSorghum sudanense)

Abstract

Products were collected from rhizospheres of a hydroponically grown sorghum-sudangrass hybrid [Sorghum bicolor(L.) Moench. ×Sorghum sudanense(P.) Stapf. ‘DeKalb SX17+’] (SSH) and tested for effects on seedling root growth of selected species. Phytotoxic activity was recovered in CH2Cl2extracts of aqueous solutions of the rhizosphere products. Significant inhibition of annual ryegrass (Lolium multiflorumL.) and alfalfa (Medicago sativaL. was produced by ca. 200 ppmw (H2O weight basis) of the most phytotoxic fraction in filter-paper-disk bioassays. In soils, 70 ppmw (soil weight basis) inhibited alfalfa but not ryegrass. Among several bioassay species, alfalfa was the most sensitive and curly dock (Rumex crispusL. ♯ RUMCR) the least. Treatment of SSH plants with herbicides increased the phytotoxicity but not the amounts of materials collected. Sethoxydim {2-[1-(ethoxyimino)-butyl]-5-[2-ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} was detected in rhizosphere products from plants treated with the herbicide. Amounts of products collected from rhizospheres of common rye (Secale cerealeL. ‘Wheeler’), johnsongrass [Sorghum halepense(L.) Pers. ♯ SORHA], and cucumber (Cucumis sativusL. ‘PI 169391’) were similar to those from SSH, but their phytotoxicities differed. Johnsongrass products were most phytotoxic, those from common rye and SSH were similar in phytotoxicity, and those from cucumber were least phytotoxic.